The invention relates to the field of power electronics. It relates to a semiconductor module according to the preamble of patent claim 1.
A semiconductor module of this type is disclosed for example in R. Zehringer et al., Materials Research Society Symposium Proceedings, Volume 483, Power Semiconductor Materials and Devices, 1998, pp. 369-380. This publication describes, as is illustrated in the accompanying FIG. 1, a semiconductor module having a module housing 1, a metallic base plate 2 and a plurality of semiconductor elements 6 which are arranged thereon and are covered by the module housing, said semiconductor elements in this case being IGBT chips (Insulated Gate Bipolar Transistor) and diodes. The module housing is generally filled with a gel composition 7, which serves as an electrical insulation layer and affords protection against corrosion and also reduces tensile forces acting on connecting wires.
The base plate is connected to a water cooling arrangement 20 in order to dissipate the heat generated by the semiconductor elements. A substrate in the form of a metal-coated ceramic plate 3 is arranged on the base plate. It forms electrical insulation between semiconductor elements and base plate or water cooling arrangement and additionally has good thermal conductivity in order to dissipate the heat from the semiconductor elements to the base plate. The base plate, ceramic plate and semiconductor elements are soldered one on top of the other, the metal layers 4, 5 of the ceramic plate enabling the soldered connection.
It is possible nowadays to combine good thermal conductivity and poor electrical conductivity in materials, with the result that producing insulation elements which are relatively thin but have good thermal conductivity, for example ones made of aluminum nitride (AlN), with a good electrical insulation capability, does not pose any difficulties. Thus, in theory, a thickness of between 1.5 and 2 mm suffices to insulate 20 kV.
Edge effects, in particular caused by edges and corners of the metal layers, have a disadvantageous effect, however, on the dielectric strength of the semiconductor module, in particular in the case of high-power semiconductor modules above 1200 V. Edges and corners of the metal layers have an inhomogeneous, intensified electric field. This excessive field increase leads to partial discharges and limits the dielectric strength of the entire construction. In this case, the field strength at the edges is at least the square of the voltage, with the result that massively thicker electrical insulation would be necessary to avoid such partial discharges. Air bubbles that may be produced precisely in the edge zones when gel is filled into the module housing promote partial discharges and form an additional critical factor with regard to the functionality of the semiconductor module.
The object of the invention, therefore is to provide a semiconductor module of the type mentioned in the introduction which has an improved dielectric strength.
This object is achieved by a semiconductor module having the features of patent claim 1.
In the case of the semiconductor module according to the invention, at least one of the metal layers is designed to be curved, as a result of which, even in edge regions, an electric field which is as homogeneous as possible is obtained and excessive field increases are minimized.
In a first embodiment, at least one of the metal layers is curved in the shape of a well, the distance from a second metal layer arranged on the opposite side of the insulation element increasing toward the edges, thereby reducing an excessive field increase in the edge region.
In a second embodiment, the curved portion is formed by bead-shaped terminations, preferably with a round cross section, at corners and/or edges. In a preferred variant, the insulation element has depressions for accommodating at least the protruding beads. If the entire metal layer is incorporated into the insulation element, with the result that it forms a plane surface with the latter, it is possible to prevent air bubbles from being formed during the gel filling process, since small cavities having poor accessibility are avoided.
Further advantageous embodiments emerge from the dependent patent claims.